1. Field of the Invention
The present invention relates generally to a low power consumption circuit and secondary battery cell protection circuit, and more particularly, to a low power consumption circuit and secondary battery cell protection circuit not easily susceptible to outside interference.
2. Description of the Related Art
Compared to the conventional nickel-cadmium battery cell or nickel-hydrogen battery, the light, compact lithium-ion battery that is one type of secondary battery cell delivers approximately three times as much operating voltage, approximately twice as much power per unit of weight and substantially greater power per unit of volume as well. The power, lightness and compactness of the lithium-ion battery cell has led to its widespread use in video cameras, portable telephones, PHS, notebook-type personal computers and a wide array of other portable electronic products.
However, over-discharge shortens the useful life of the lithium-ion battery cell, and moreover, the lithium-ion battery cell cannot be reused once it is completely discharged. On the other hand, continuing to charge the lithium-ion battery cell to a state of overcharge increases the cell voltage and poses a hazard, also shortening the useful life of the lithium-ion battery cell.
Therefore, in order to ensure the safety and enhance the performance of these light, compact but powerful battery cells, protection circuits are used.
FIG. 1 is a block diagram for the purpose of explaining a conventional secondary battery cell protection circuit for a secondary battery cell such as a lithium-ion battery cell. The circuit has either a charger or a load connected between +B power terminal and -B power terminal. When a charger is connected to the power terminals, the lithium-ion battery cell is charged, and when a load is connected to the power terminals, the lithium-ion battery cell is discharged.
The secondary battery cell protection circuit for a lithium-ion battery cell has a secondary battery cell protection circuit 2, a power line 5 that connects the secondary battery cell 1 and the +B power terminal, a resistor 3 connected between the power line 5 and the secondary battery cell protection circuit 2, a resistor 4 connected between the secondary battery cell protection circuit 2 and a power line 12 that is connected to the -B power terminal, discharge control field-effect transistor (hereinafter FET) FET #1 and charge control FET #2.
The secondary battery cell protection circuit 2 comprises over-discharge detection circuit 13 for detecting an over-discharge, overcharge detection circuit 14 for detecting an overcharge, overcharge control circuit 15 for applying a control signal to a terminal 5 and cutting off charge control FET #2 when the overcharge detection circuit 14 detects an overcharge, and an over-discharge control circuit 16 for applying a control signal to a terminal 3 and cutting off over-discharge control FET #1 when the over-discharge detection circuit 13 detects an over-discharge.
Additionally, the secondary battery cell protection circuit 2 comprises terminals 1, 2, 3, 4, 5. Terminal 2 is a terminal for detecting an electric potential at a negative side of the secondary battery cell 1, terminal 4 is a terminal for detecting an electric potential at the power line 12 connected to the -B power terminal. The secondary battery cell protection circuit 2 is constructed so that the circuitry involved with over-discharge control and overcharge control is activated whenever the electric potential at terminal 4 is lower than the electric potential at terminal 2.
Terminal 1 is a terminal for detecting an electric potential at a positive side of the secondary battery cell 1. Based on the electric potential at this terminal, the over-discharge detection circuit 13 and the overcharge detection circuit 14 carry out a determination as to the over-discharge state or overcharge state of the secondary battery cell 1. If the electric potential falls below a predetermined electric potential and the over-discharge detection circuit 13 detects an over-discharge, then the over-discharge control circuit 16 switches terminal 3 to LOW level and cuts off over-discharge control FET #1. If, on the other hand, the electric potential at terminal 1 rises above a predetermined electric potential and the overcharge detection circuit 14 detects an overcharge, then the overcharge control circuit 15 switches terminal 3 to LOW level and cuts off overcharge control FET #2. By so doing, the secondary battery cell 1 can be protected from both excessive charge and excessive discharge.
However, consider a state in which a load is connected between the +B power terminal and the -B power terminal of the circuit shown in FIG. 1, with the secondary battery cell 1 supplying power to the load.
When the secondary battery cell 1 reaches an over-discharge state as a result of supplying power to the load for a long period of time, the electric potential at terminal 1 drops and the over-discharge detection circuit 13 detects an over-discharge. When an over-discharge is detected the over-discharge control FET #1 is cut off by the over-discharge control circuit 16. At the same time, in order to prevent damage to the secondary battery cell 1, the secondary battery cell protection circuit 2 is shut off except for a portion of the secondary battery cell protection circuit 2, thus halting operation of the secondary battery cell protection circuit 2. As a result, high impedance arises at the power line 12 between the -B power terminal and the FET #1.
However, the high-impedance power line 12 is very susceptible to the effects of external noise or interference, caused, for example, by the placement of a finger on the -B power terminal or power line 12, as a result of which the electric potential at the power line 12 fluctuates. That fluctuation in the electric potential can cause the electric potential at terminal 4 to drop below the electric potential at terminal 2, activating the discharge control and charge control circuits of the secondary battery cell protection circuit 2 as a result.
The activation of the discharge control and charge control circuits of the secondary battery cell protection circuit 2 circuit that the secondary battery cell protection circuit 2 is mistakenly activated by external interference despite the operation of the secondary battery cell protection circuit 2 having been halted in order to protect the secondary battery cell 1 from damage due to over-discharge of the secondary battery cell 1.